With wide application of grid-connected power generation technologies, grid companies pay more attention to quality of electric energy outputted by a grid-connected inverter. To reduce harmonics of grid-connected output and improve quality of electric energy, strict requirements for content of harmonics of respective orders and total harmonic content in a grid-connected current have been raised in Chinese and foreign photovoltaic power generation standards (for example, UL1741, IEEE 929-2000). There are many factors which affect the harmonics in the grid-connected current, and the influence of a manner for filtering the AC output of the grid-connected inverter on the harmonics in the grid-connected current is mainly discussed in the present invention.
At present, output filters used in grid-connected inverter are all designed according to the rated output power of the grid-connected inverter. When the output power of the grid-connected inverter is the rated output power, the harmonic distortion of the output current waveform of the grid-connected inverter can meet the requirements of relevant standards of the grid-connected inverter well. However, in the case of light load, especially when a load factor is lower than 25% and the output power of the inverter is far lower than the rated output power, the following two serious problems will occur:
(1) In the case of low load factor, the harmonic content of the grid-connected current exceeds the standards severely due to low modulation ratio and low switch duty ratio, which leads to great harmonic pollution to the power grid.
(2) In the case of light load, the conversion efficiency of the inverter is reduced largely due to the increase of the proportion of no-load loss of the filters.
Such problems are especially serious for a photovoltaic grid-connected inverter. Due to natural change of sunshine intensity in a day, the photovoltaic grid-connected inverter keeps being operated at a lower load factor during most of the day. In this case, as described above, the photovoltaic grid-connected inverter has the problems that the harmonic content of the grid-connected current exceeds the standards and the conversion efficiency of the inverter is reduced, so that quality of the outputted electric energy is degraded. However, a solution to such problems is still absent up to now.